We studied the genetic variation among subpopulations of cultured HeLa and RAT-1 cells with the DNA fingerprint technique. The technique makes it possible to screen for differences among scores of loci scattered throughout the genome. We compared the fingerprint patterns of mass populations of HeLa cells with those of individual clones selected from the populations. We found differences in band patterns in 3% of the loci examined. We then made similar comparisons with RAT-1 cells, finding no differences in any of the bands studied.